1. Field of the Invention
This invention relates generally to a method and apparatus useful in the exploration of subterranean regions and, more particularly, to a method and apparatus useful in the exploration for, and production of, hydrocarbons from subterranean regions. In various aspects, the present invention relates to a method and apparatus for a new type of wire that combines high conductivity and low thermal expansion that can be used to reduce temperature sensitivity effects in induction logging tools and other instruments that require wires with low thermal expansion and low electrical resistance.
2. Description of the Related Art
As shown in FIG. 1, a conventional drilling and/or logging operation is shown, including a drilling and/or logging rig 120 and a downhole induction logging tool 100. The drilling and/or logging rig 120 is generally a rotary drilling and/or logging rig of the type that is well known in the drilling and/or logging art and comprises a mast 122 that rises above the ground 124. The rotary drilling and/or logging rig 120 is fitted with lifting gear (not shown) from which is suspended a drill and/or logging string 126 formed by a multiplicity of drill pipes 128 screwed into one another, and/or a logging wire or cable (not shown), where the drill and/or logging string 126 may have at its lower downhole end a drill bit 132 for the purpose of drilling a well bore 134. The downhole induction logging tool 100 may be located in the drill and/or logging string 126 in any suitable location and by any suitable manner known to those in the relevant art.
Induction tool measurements used in logging applications, made by the downhole induction logging tool 100, for example, are very sensitive to changes in the position and/or the diameter of either or both the receiver and/or the transmitter coils. The dimensions of all the parts of an induction tool should be as invariant as possible even as the temperature of the induction tool varies over a wide range from typical surface temperatures of about 68° F. (20° C.), or lower (sub-zero surface temperatures could be experienced, for example, during testing and calibration in Alaska, for example), to downhole temperatures of about 500° F. (260° C.), or higher (generally, the deeper the well, the higher the downhole temperature), in the deeper wells. Low thermal expansion materials have typically been used, such as low thermal expansion ceramics, for example, silicon nitride (Si2N3).
The wire in the coils of the induction tools are typically made of thin copper (Cu) filaments. Copper (Cu) is selected for its low electrical resistance (high electrical conductivity). However, copper (Cu) is less than optimal for these induction tool applications due to its relatively high thermal expansion. The effect of the thermal expansion of copper (Cu) can produce significant changes in the signals measured, particularly in the shallower induction measurements that are associated with shorter receiver-transmitter spacings.
As shown in FIGS. 2 and 3, a type of wire conventionally used in downhole induction tools is a litz wire 200 made up of many fine, and separately insulated, strands 210 of copper (Cu) that are woven together, typically so that each of the strands 210 successively takes up all possible positions in the cross-section of the litz wire 200. FIG. 2 shows a side view of the litz wire 200. FIG. 3 shows a cross-sectional view of the litz wire 200.
The purpose of the strands 210 is to provide the maximum surface area for a given cross-section of the litz wire 200. At the typical frequencies used in downhole induction tools, in a range from about 8 kHz to about 200 kHz, the electrical current flows in a thin layer at or near the surface of each of the strands 210 of the litz wire 200. By using several strands 210, the overall resistance of the litz wire 200 is reduced at these typical frequencies. However, if no method to reduce the thermal expansion of the litz wire 200 is used, the thermal expansion of the litz wire 200 with increased temperature produces changes in the signal level of the downhole induction tool 100 that are tolerable for receiver-transmitter induction coil spacings greater than about 20 inches (50 cm), but that are not tolerable for receiver-transmitter induction coil spacings less than about 20 inches (50 cm). For receiver-transmitter induction coil spacings less than about 20 inches (50 cm), the effect of the thermal expansion of the litz wire 200 with increased temperature is very significant, producing intolerable changes in the signal level of the downhole induction tool 100, making the achievement of more stable measurements much more difficult.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.